The indoor radon distribution, presented as “mountains”.
This map was produced from the same radon information by zip
code mapped in blue, red and yellow at our
website home page. Altitude
points, representing percentages of homes exceeding the EPA guideline
for indoor radon, have been placed at the centers of zip code regions,
and then processed into a relief map, shown with shading as if the sun
were to the north-west.
the map of geology and radon, can you see one or two associations of
indoor radon with certain bedrock types?
radon measurement summaries used for
this map, one can see that elevated indoor radon is found in every
region of Wisconsin, more in some regions than in others.
radon mountains, based on zip-code data summarized as altitudes at zip
centroids, are a crude approximation of the actual indoor radon
distribution. But they
have the advantage that they can be shown in one map along with the
bedrock type, allowing for direct visual perception of some
associations. One can see
zip code boundaries in the color
map of indoor radon by zip-code.
Zip codes are better for this purpose than one would guess at
first, because regions of high population and high data density tend
to have small zip-code areas, and regions of low population and low
data density tend to have larger zip-code areas, causing the
statistics for each altitude point to be better than they would be
using a grid of uniform areas.
Underlying bedrock is an indirect
factor, and just one of many factors that can affect indoor radon
a gas, can diffuse only a meter or two through soil from where it is
created by radium decay, before it is transformed into a chemically
reactive atom by its own radioactive decay. The soil under and around a house is usually the source of the
Soils are derived by weathering of bedrock
through eons of time.
Some soils have been transported by glacial
action from the bedrock where they initially developed.
Radon comes from uranium through a long series of
radioactive transformations. Uranium
itself (half-life 4.5 billion years) is primordial, created in
stellar supernova explosions along with many other elements (iron,
nickel, silicon etc.) in interstellar dust from which the earth
Although rock types tend to have characteristic
uranium concentrations, the uranium content can vary from place to
place within a rock type.
Uranium, radium and other elements in the decay
chain can be moved geochemically, for example by dissolving in
water and concentrating where they come out of solution.
House construction (openings to soil) is another
variable factor of indoor radon.
for the maps: Prof.
Michael G. Mudrey, Wisconsin
Geological and Natural History Survey, retired